Metasomatic albitites and related biotite‐rich schists from a low‐pressure polymetamorphic terrane, Snake Creek Anticline, Mount Isa Inlier, north‐eastern Australia: microstructures and P–T–d paths

Abstract

Rocks of the Snake Creek Anticline are mainly pelitic schists, psammitic schists and quartzites that were metamorphosed during multiple high‐T/low‐P events extending from D1 to D5, with the metamorphic peak occurring late to post‐D3. Albitites are widespread, but are concentrated in five areas. They are typically fine‐ to medium‐grained, and consist of albite, with or without combinations of quartz, biotite, staurolite, cordierite, garnet, andalusite, sillimanite, kyanite, gedrite and tourmaline. From the presence or absence of albite inclusions in porphyroblasts, the albitites are interpreted as forming early in the D3 event as a result of infiltration of external fluids. Psammitic schists and quartzites were preferentially altered, but pelitic schists were also albitized in localities where the alteration was more extreme, with the replacement of muscovite total and the replacement of quartz and biotite variable. Structural controls on albitization include fracturing and syn‐D3 shear zones in fold hinges. Biotite schists with abundant porphyroblasts (combinations of staurolite, garnet, andalusite and cordierite) occur adjacent to albitites, and it is argued that they formed by the addition of Fe and Mg sourced from the albitites. In several albitite‐rich areas, cordierite grew early in D3 and was partly or entirely replaced during or after D3 by combinations of biotite, andalusite, tourmaline, staurolite and sillimanite. A postulated P–T–d path involved an increase in pressure (with or without a decrease in temperature) subsequent to early D3 albitization, followed by an increase in temperature up to the metamorphic peak (late D3 to early D4. The metamorphism was contemporary in part with the emplacement of the Williams Batholith (c. 1550–1500 Ma), which probably supplied the Na‐rich fluids.

Metamorphic evolution of the Georgetown Inlier, northeast Queensland, Australia; evidence for an accreted Palaeoproterozoic terrane?

“…Another feature thought to be common to the Mt Isa and Georgetown Inliers was anomalously high heat flow, expressed via bimodal magmatism and low‐pressure high‐temperature metamorphism which initially preceded, but then accompanied convergent orogenesis. A low‐pressure high‐temperature P–T evolution is well established in the Mt Isa Inlier (Reinhardt, 1992; Rubenach, 1992; Rubenach & Barker, 1998; Rubenach & Lewthwaite, 2002). In this region, the cause of the anomalously high geothermal gradients (40–60 °C km −1 ) are explained via a number of mechanisms.…”

Dating Metasomatism: Monazite and Zircon Growth during Amphibolite Facies Albitization

“…Albite is a common metasomatic product of Na-rich fluids [17,57,58] moving through the crust and altering granitoids. During this alteration process, quartz can be consumed to facilitate the transformation of Al-rich anorthite feldspar into Si-rich albite (e.g., [17,59]).…”

Mesoproterozoic plume‐modified orogenesis in eastern Precambrian Australia

“…In the Mount Isa Inlier, the Isan Orogeny [ Bell , 1983; Betts et al , 2006; Blake and Stewart , 1992; O'Dea et al , 1997b; Page and Bell , 1986] has been interpreted to span more than 100 Ma (Figure 10) but most likely represents multiple orogenic events [ Betts et al , 2000, 2006; Giles et al , 2006a; O'Dea et al , 2006; Sayab , 2008] with several discrete episodes of metamorphism [ Connors and Page , 1995; Foster and Rubenach , 2006; Giles et al , 2006b; Giles and Nutman , 2002, 2003; Page and Sun , 1998; Rubenach , 1992; Rubenach and Barker , 1998; Rubenach and Lewthwaite , 2002]. In the relatively low strain regions of the Western Fold Belt the early stages of the Isan Orogeny involved inversion of the rift‐sag basins during both north‐south and east‐west shortening resulting in normal fault reactivation, development of localized foliations and development of north to northeast trending upright folds (Figure 7) [ Betts et al , 2004; Lister et al , 1999; O'Dea and Lister , 1995; O'Dea et al , 1997a].…”